Thermal selectivity multivariate optical computing

ABSTRACT

A method of using photoacoustic spectroscopy to determine chemical information about an analyte includes the steps of emitting a light ray for interaction with a sample of an analyte; transmitting the light ray through a fill fluid disposed in a detection cell, the fill fluid having molecules substantially similar to molecules of the analyte to absorb the light ray; producing a thermal wave and oscillation in the fill fluid proportional to an intensity of the light ray; including a pressure oscillation in the fill fluid by the thermal wave; and detecting the pressure oscillation by a microphone to determine information about the analyte sample.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of, and claims priority to, U.S. patentapplication Ser. No. 11/576,359, entitled “Thermal SelectivityMultivariate Optical Computing,” filed Jul. 14, 2008 now U.S. Pat. No.8,240,189, which was the National Stage of International Application No.PCT/US2005/035617, file Oct. 4, 2005, which claims the benefit of U.S.Provisional Application Ser. No. 60/615,808, entitled “ThermalSelectivity Multivariate Optical Computing,” filed Oct. 4, 2004, each ofwhich is hereby incorporated by reference in their entirety.

STATEMENT OF FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under grant numberF33615-00-2-6059 awarded by the United States Air Force ResearchLaboratory. The Government has certain rights in this invention.

FIELD OF THE INVENTION

The present invention relates to filters that improve the precision ofmultivariate optical computing in spectroscopic analysis.

BACKGROUND OF THE INVENTION

The present invention relates to filters that improve the precision ofmultivariate optical computing Multivariate optical computing (MOC) is apredictive spectroscopy technique that incorporates a multi-wave-lengthspectral weighting directly into analytical instrumentation. Themeasurement precision of MOC has been studied for various techniques,several of which involve the use of an interference filter described asa multivariate optical element (MOE). Since MOE-based MOC uses detectorsthat see all wavelengths simultaneously—including wavelengths that carryno information—measurement noise is reduced and measurement precision isincreased if the system can be made sensitive primarily to wavelengthscarrying information.

In absorption/transmission/reflection measurements, the best measurementprecision occurs when the detector responds only to those wavelengthswhere the sample analyte exhibits absorption. Thus, the ideal detectorresponse would be one that only accumulates a signal where variancerelated to the analyte occurs in a data set. Such detection schemes arepossible with thermal measurements as compared to purely optoelectronicdetectors.

BRIEF SUMMARY OF INVENTION

The present invention is generally directed to thermal-based detectorsthat improve the precision of multivariate optical computing forspectroscopic analysis.

One type of thermal detection according to an aspect of the invention isbased on photoacoustic spectroscopy. Photoacoustic detection offers ahighly sensitive measurement scheme in the mid-infrared (MIR) farliquids, solids and gases by observing the thermal-wave decay ofabsorption-induced beating via a pressure oscillation in a sample cell.A photoacoustic detector using a fill gas (or liquid) composed of theanalyte alone (or with a non-absorbing filler) will provide a detectorthat is sensitive primarily to the wavelengths absorbed by the analyte.The effective wavelength range utilized in a measurement using such adetector will only be of those wavelengths where the sample exhibitsabsorption-induced heating. Effectively, the analyte becomes thedetector, and the multivariate optical element is designed to correlatethe spectral variance in the photoacoustic signal with the analyteconcentration. The photoacoustic cell can be filled with analyte to aconcentration that provides optimal signal to noise, since the analytefilling the cell is not the sample being measured. Instead, thephotoacoustic-analyte cell is used only as a detector for light passingthrough, emitted by, scattered from, or reflecting from the sample.

Among the additional types of detectors made for thermal selectivity arepyroelectric detectors, thermoelectric or thermopile detectors,bolometers. In each case, a reflective coating (for example, a goldmetal film) on the original detector can be used to eliminate the broadwavelength sensitivity of the detector, while a polymer or liquidcoating can be applied in a thin film atop the reflector to givesensitivity to analyte absorption bands.

In one aspect of the invention, a method of using photoacousticspectroscopy to determine chemical information about an analyte includesthe steps of emitting a light ray for interaction with a sample of ananalyte; transmitting the light ray through a fill fluid disposed in adetection cell, the fill fluid having molecules substantially similar tomolecules of the analyte to absorb the light ray; producing a thermalwave and oscillation in the fill fluid proportional to an intensity ofthe light ray; the thermal wave inducing a pressure oscillation in thefill fluid; and detecting the pressure oscillation by a microphone todetermine information about the analyte sample. In this aspect, theanalyte can be a gaseous analyte, a liquid analyte, a solid analyte, adissolved analyte, a powdered analyte, an emulsified analyte, orcombinations of these analytes. The analyte molecules can also be thesame as the fill fluid molecules in this aspect.

Also in this aspect of the invention, the analyte can include anon-absorbing carrier fluid to absorb the light ray, which can beemitted from a broadband light source. The light ray interacts with theanalyte in this aspect by being emitted through, emitted by, scatteredfrom, or reflecting from the analyte.

Further in this aspect of the invention, the method can include the stepof modulating an intensity of the light ray at a fixed frequency. Afurther step according to the method is to read a detector response atthe fixed frequency. A wavelength of the light ray in this aspect caninclude a non-zero spectral weighting. Also in this aspect, themicrophone records only the pressure oscillation induced by an absorbedwavelength of the light ray.

According to another aspect of the invention, a photoacoustic detectionsystem includes a detection cell having a chamber and a port definedtherein. The chamber is configured to hold a fill fluid and analyte, anda microphone detector is located in the port, the microphone detectorbeing configured to record a pressure oscillation in the till fluidinduced by the wavelengths of a light ray absorbed by the fill fluidsuch that information about the analyte can be determined. The pressureoscillation detected by the microphone detector provides informationabout the analyte.

The detection cell in this aspect of the invention includes an inlet forinjecting the fill fluid and the analyte into the chamber. Further, thedetection cell includes an outlet for releasing the fill fluid and theanalyte franc the chamber.

Also in this aspect of the invention, the photoacoustic detection systemcan include a light source being configured to emit the light ray in adirection of the chamber. The light source can be a broadband lightsource. The light source can be configured to modulate an intensity ofthe light ray at a fixed frequency.

Further in this aspect of the invention, the photoacoustic detectionsystem can include an oscilloscope in communication with the microphonedetector, the oscilloscope being configured to present a system responsebased on the pressure oscillation. The system response is produced by awavelength of the light ray with a non-zero spectral weighting.

Other aspects and advantages of the invention will be apparent from thefollowing description and the attached drawings, or can be learnedthrough practice of the invention.

BRIEF DESCRIPTION OF DRAWINGS

A full and enabling disclosure of the present invention, including thebest mode thereof to one of ordinary skill in the art, is set forth moreparticularly in the remainder of the specification, including referenceto the accompanying figures in which:

FIG. 1 is a schematic view of a photoacoustic infrared cell in a FourierTransform instrument for measuring calibration spectra and collecting aphotoacoustic signal according to an aspect of the invention;

FIG. 2A is a graph comparing a system response to a regression vector ofa multivariate optical element that would produce measurements with poorprecision because the spectral sensitivity overlaps the spectral detailspoorly;

FIG. 2B is a graph similar to FIG. 2A showing a better detector responsethat would produce modest precision because, while sensitive to theimportant spectral window, the system also accumulates signal frontwavelengths outside the important region;

FIG. 2C is a graph similar to FIG. 2A showing an ideal system responsewhere only the wavelengths with a non-zero spectral weighting producethe system response according to another aspect of the invention; and

FIG. 3 illustrates the steps of the method of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Detailed reference will now be made to the drawings in which examplesembodying the present invention are shown. Repeat use of referencecharacters in the drawings and detailed description is intended torepresent like or analogous features or elements of the presentinvention.

The drawings and detailed description provides full and detailed writtendescription of the invention and the manner mid process of making andusing it, so as to enable one skilled in the pertinent art to make anduse it. The drawings and detailed description also provide the best modeof carrying out the invention. However, the examples set forth hereinare provided by way of explanation of the invention and are not meant aslimitations of the invention. The present invention thus includesmodifications and variations of the following examples as come withinthe scope of the appended claims and their equivalents.

Multivariate optical computing (MOC) according to some aspects of thepresent invention simplifies instrumentation and data analysisrequirements of multivariate calibration. As the figures generally show,a multivariate optical element (MOE) utilizes a thin film interferencefilter to sense magnitude of a spectral pattern. A no-moving-partsspectrometer, which is highly selective to a particular analyte, can beconstructed by designing simple calculations based on the filtertransmission and reflection spectra. A high throughput measurement canalso be made since a broadband light source is used and many wavelengthsare seen simultaneously at the detector.

Turning now to FIG. 1, a photoacoustic detection system is designated ingeneral by the element number 10. The photoacoustic detection system 10broadly includes a dual function, infrared gas cell or photoacousticdetection cell 12, a carrier fluid such as a gas 14, an analyte such asa vapor 15, a light source 16 and an oscilloscope 18. The dual functionarises from use of a single cell to measure both light transmissionspectra and photoacoustic spectra. More specifically, as described indetail below, the photoacoustic detection cell 12 may be used formeasuring calibration spectra in a Fourier Transform instrument as wellas for collecting a final photoacoustic signal regarding the vapor 15.Although the analyte 15 is a vapor in this example, this embodiment ofthe invention is not limited to a gaseous analyte. The analyte 15 can bea liquid analyte, a solid analyte, a dissolved analyte, a powderedanalyte, or an emulsified analyte, or combinations of these.

Turning to one function of cell 12, i.e., as a photoacoustic detectioncell, the photoacoustic detection cell 12 of FIG. 1 includes a pluralityof surfaces and walls 12 A-D that form an interior or chamber 12 E. Asshown, a carrier gas & vapor inlet port 20, a carrier gas and vaporoutlet port 22 and a central port 24 are formed in the surface 12 A. Theskilled artisan will appreciate that the number and shape of thesurfaces and walls 12 A-D and the locations of the ports 20, 22 and 24can be modified to suit various applications and are not limited to theexample shown, and that detection can be performed with other tools thanthe oscilloscope 18.

FIG. 1 further shows that a microphone detector 26 is arranged in thecentral port 24, and one or more glass plates 28 are located in and helpform the end walls 12C, 12D. As shown, the carrier gas 14 and theanalyte vapor 15 are introduced into the chamber 12E via the carrier gas& vapor inlet port 20. The microphone detector 26 is connected to theoscilloscope 18 to record and analyze radiometric and photoacousticmeasurements based on interaction a light rays or beams 32 with aninterface or gaseous matrix 30 formed by the carrier gas 14 and theanalyte vapor 15, as described in greater detail below.

FIGS. 2A-C generally show measurement precision examples comparing asystem response to a T-R regression vector of a multivariate opticalelement, where T-R is a regression estimator (t_(r)).

Aspects of the invention may be better understood with reference toFIGS. 1-2C and to an exemplary method of operation.

Turning to the other function of cell 12, i.e., to measure transmissionspectra of light rays, as shown in FIG. 1 and briefly introduced above,the dual function infrared gas cell 12 can be designed to be inserteddirectly into a spectrometer (for example, a FT-infrared spectrometer)so that the system response of this detector can be determinedprecisely.

As briefly introduced, photoacoustic spectroscopy is a specific thermal,non-destructive measurement of the interaction between the incidentlight rays 32 and the carrier gas 14 and the interface 30 via anon-radiative relaxation process. As shown in FIG. 1, the incident lightrays 32 are emitted by the pulsed or chopped light source 16, which inthis example is a broadband light source. The light passes through asample 35, containing at least one gaseous analyte to be measured. Afterpassing through the sample, the light rays 32 interact with the MOE,then enter the photoacoustic cell 12. The cell 12 is filled with agaseous mixture of vapor of the same molecular type as the analyte 15,with possibly an inert/absorption-free carrier gas 14. The fill gasabsorbs the incident light rays 32. This interaction produces a thermalwave 34 or heat proportional to the intensity of the incident beam 32,which oscillations can he detected by the microphone 26. By modulatingthe source 16 and thus the intensity of the incident radiation 32 at afixed frequency, the detector response can be read at the referencefrequency yielding a very precise measurement. Ultimately, theanalyte-rich fill gas 15 becomes the detector since the microphone 26only responds to those wavelengths that were absorbed by the fill gas15.

The skilled artisan will understand that photos cons-tic detection isnot limited to gases, but can be applied to liquids and even solids.Other types of thermal detectors can also be used to measure gases,liquids and solids by the method described here.

Turning now to FIG. 2A, poor precision is shown because there is littleoverlap between a system response 36 and a regression vector 38. Incomparison, FIG. 2B shows moderate precision since the system response136 is also accumulating a signal from wavelengths outside of aregression vector 138. In FIG. 2C, an ideal system response according tothis aspect of the invention is shown in which only the wavelengths witha non-zero spectral weighting produce a system response 236, whichcorrelates substantially with a regression vector 238.

Turning to FIG. 3, the steps of preferred methods of the invention areillustrated. In a method 300 for using photoacoustic spectroscopy todetermine chemical information about a sample analyte, in step 302, alight ray is emitted for interaction with the sample analyte. In step304, the light ray is passed through a multivariate optical element. Instep 306, the interacted light ray from the sample is transmitted into adetection cell. The detection cell includes a fluid disposed therein andselected to have molecules of the sample analyte disposed to absorb thetransmitted light ray. In step 308, a thermal wave and oscillation isproduced in the fill fluid. The thermal wave is proportional to anintensity of the light ray. In step 310, a pressure oscillation in thefill fluid is produced by the thermal wave. Finally, in step 312, thepressure oscillation is detected by a microphone to determineinformation about the analyte.

While preferred embodiments of the invention have been shown anddescribed, those skilled in the art will recognize that other changesand modifications may be made to the foregoing examples withoutdeparting from the scope and spirit of the invention. It is intended toclaim all such changes and modifications as fall within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A photoacoustic detection system, comprising: alight source to emit a light ray that optically interacts with a samplecomprising an analyte to be measured; a multivariate optical elementthrough which the light ray passes; a detection cell having a chamberand a port defined therein, the chamber being configured to hold a fillfluid and an analyte, wherein the fill fluid analyte is the same as thesample analyte; and a microphone detector disposed in the port, themicrophone detector being configured to record a pressure oscillation inthe fill fluid induced by the wavelengths of the light ray absorbed bythe fill fluid such that information about the sample analyte can bedetermined.
 2. The photoacoustic detection system as in claim 1, whereinthe detection cell includes an inlet defined therein, the inlet beingconfigured to inject the fill fluid and the analyte into the chamber. 3.The photoacoustic detection system as in claim 1, wherein the detectioncell includes an outlet defined therein, the outlet being configured torelease the fill fluid and the analyte from the chamber.
 4. Thephotoacoustic detection system as in claim 1, wherein the pressureoscillation detected by the microphone detector provides informationabout the sample analyte.
 5. The photoacoustic detection system as inclaim 1, wherein the light source is a broadband light source.
 6. Thephotoacoustic detection system as in claim 1, wherein the light sourceis configured to modulate an intensity of the light ray at a fixedfrequency.
 7. The photoacoustic detection system as in claim 1, furthercomprising an oscilloscope in communication with the microphonedetector, the oscilloscope being configured to present a system responsebased on the pressure oscillation.
 8. The photoacoustic detection systemas in claim 7, wherein the system response is produced by a wavelengthof the light ray with a non-zero spectral weighting.
 9. Thephotoacoustic detection system as in claim 1, wherein the fill fluidcomprises at least one of a gaseous analyte, a liquid analyte, a solidanalyte, a dissolved analyte, a powdered analyte, or an emulsifiedanalyte.
 10. The photoacoustic detection system as in claim 1, whereinthe fill fluid comprises a substantially non-absorbing carrier fluid.11. The photoacoustic detection system as in claim 1, wherein thedetection cell is a single dual function detection cell that operates asan infrared gas cell or photoacoustic detection cell.